The mechanism of action of suppressor cells from mice tolerant to human-gamma-globulin will be determined by using adoptive transfer experiments, the in vitro antibody response, and an in vitro antigen induced proliferation assay. These experiments will be aimed at determining which population of cells is responsible for the transfer of active suppression from tolerant mice to normal mice to the tolerance inducing antigen. We wish to determine the cellular site of action of suppressor cells and suppressor factor from tolerant mice with particular emphasis on whether regulation occurs at the level of macrophages, B-lymphocytes and/or T-lymphocytes. Experiments will be done to determine the genetic restrictions for the action of suppressor factor from tolerant mice with respect to the donor-recipient histocompatibility requirements for suppressor factor activity. Lastly, experiments will be aimed at generating suppressor factor from tolerant spleen cells and helper factor from primed t-cells, all with specificity for human-gamma-globulin, in an attempt to detemine the functional and chemical relationship of these immunoregulatory molecules.